Subject: Re: [PATCHv7 3/3] vhost_net: a kernel-level virtio server

From: Gregory Haskins <[email-masked]>
Date: Tue, 03 Nov 2009 13:14:06 -0500

Gregory Haskins wrote:
> Eric Dumazet wrote:
>> Michael S. Tsirkin a écrit :
>>> +static void handle_tx(struct vhost_net *net)
>>> +{
>>> +	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_TX];
>>> +	unsigned head, out, in, s;
>>> +	struct msghdr msg = {
>>> +		.msg_name = NULL,
>>> +		.msg_namelen = 0,
>>> +		.msg_control = NULL,
>>> +		.msg_controllen = 0,
>>> +		.msg_iov = vq->iov,
>>> +		.msg_flags = MSG_DONTWAIT,
>>> +	};
>>> +	size_t len, total_len = 0;
>>> +	int err, wmem;
>>> +	size_t hdr_size;
>>> +	struct socket *sock = rcu_dereference(vq->private_data);
>>> +	if (!sock)
>>> +		return;
>>> +
>>> +	wmem = atomic_read(&sock->sk->sk_wmem_alloc);
>>> +	if (wmem >= sock->sk->sk_sndbuf)
>>> +		return;
>>> +
>>> +	use_mm(net->dev.mm);
>>> +	mutex_lock(&vq->mutex);
>>> +	vhost_no_notify(vq);
>>> +
>> using rcu_dereference() and mutex_lock() at the same time seems wrong, I suspect
>> that your use of RCU is not correct.
>>
>> 1) rcu_dereference() should be done inside a read_rcu_lock() section, and
>>    we are not allowed to sleep in such a section.
>>    (Quoting Documentation/RCU/whatisRCU.txt :
>>      It is illegal to block while in an RCU read-side critical section, )
>>
>> 2) mutex_lock() can sleep (ie block)
>>
> 
> 
> Michael,
>   I warned you that this needed better documentation ;)
> 
> Eric,
>   I think I flagged this once before, but Michael convinced me that it
> was indeed "ok", if but perhaps a bit unconventional.  I will try to
> find the thread.
> 
> Kind Regards,
> -Greg
> 

Here it is:

http://lkml.org/lkml/2009/8/12/173

Kind Regards,
-Greg